Switching system



June 4,1940. r J, m; 2,203,557

SWITCHING SYSTEM Filed July 28, 1938 IOSheets-Sheet 1 T0 CONTROL APPARATUS T0 CONTROL/APPARATUS FIG. 1

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INVENTOR.

JOHN WICKS ma mm ATTORNEYS.

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June 4, 1940. J, w cKs SWITCHING SYSTEM led Jul 28, 1938 10 Sheets-Sheet 7 NmNGm INF/bi ATTORNEYS June 4, 1940. I .J. wlcKs SWITCHINGISYSTEM Filed July 28, 1958 10 Sheets-Sheet 8 Loom IN VENTOR.

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issfi i 10 Sheets-Sheet 9 J. wlcKs swncame SYSTEM Filed July 28, 1938 JOHN WICKS W, M

ATTORNEYS.

June 4,.,1940.

June 4, 1940. WICKS SWITCHING SYSTEM ATTORNEYS .30 ing bars are provided for moving the correspond communication connection and wherein the com- 30 "45 multiplied, it is possible to select any desired station embodied in the system; 45 line from a group of lines either incoming to or The features of the invention as described in outgoing from the switch. detail hereinafter are shown as being embodied Patented June 4, 1940 I p UNITED. STATES PATENT OFFlCE SWITCHING SYSTEM John Wicks, Oak Park, Ill., assignor to Associated Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application July 28, 1938, SerialNo. 221,722

I as Claims. 01. 179--18) The present invention relates to automatic switches involved, is conventionally made responswitching systems and more particularly to sive to impulses transmitted thereto from a callswitching systems of the character used in autoing subscribers substation when the apparatus inatic telephone systems. is taken for use. The arrangement of this control.

It is wellgknown that connections between the apparatus may take any one of various forms. lines of an automatic telephone system are estab- In order to simplify and economize in the matter lished by routing the connections from the callof the apparatus provided in conjunction with ing lines by way of automatically operating trafeach of the switches per, se, there has been fic concentration switches and one or more tan: proposed in the past a form of system wherein 1U dcm connected impulse responsive numerical separate communication and setting routes are switches which are operative to select the deutilized in setting up the switch trains by way sired lines and to establish connections thereto. of which established connections are routed. An

Various types of automatic switches for perform improved form of this type of system is disclosed I ing the traffic concentration and line selecting in applicants prior Patent No. 2,081,027, granted r functions are known to, the art. In applicants May 18, 1937. As described in this patent, the prior Patent No. 2,137,517, granted Nov. 22, 1938, switches involved in the setting route are autothere is disclosed an improved form of switch matically released for further use immediately which, because of its construction and mode of following their-operation to cause the selection operatiQn is well adapted for use as a nonnu and operation of selected ones of the switches merical switch or as an impulse responsive nuembodied in the communication channel. By 20 merical switch. This improved switch, which this arrangement, it is possible not only to simfalls broadly into the class known as cross bar plify the actual construction of the switches Which switches, comprises a plurality of members or are tied up in the communication switch trains, bars divided into two sets which may arbitrarily but also greatly to reduce the number of switches be designated as comprisingvertical bars and required in the setting channel. 25

horizontal bars. Associated with each of the bars It is an object of the present invention to proand adapted to be moved thereby are a plurality vide an improved automatic switching system of sets of contacts. Actuating means comprising of the type wherein separate setting and commumagnets individually associatedwith the operatnication channels are utilized in setting up a ing bars from one position to a second position. munication switches are ofthe cross bar type.

The bars of one of the twogroups, such, for It is a further object of the invention to proexample, asthe vertical bars, are so arranged vide an improved arrangement of the character relative to the bars in the other group, namely described wherein the conversational switches the horizontal bars, that, when a selected one of are positively controlled in a simple and reliable 35 the magnets associated with one of the horimanner through the setting or by-pass switches. zontal and the magnet associated with a selected It is a still further object of the invention to one of the vertical bars are energized, the conprovide an automatic telephone system of the tacts of asingle set of contacts associated with character indicated wherein the by-pass or setthe operated vertical bar are moved into engageting switches are arranged in an improved man- 40 ment with the contacts ofa single set of contacts nor to enable the operating magnets of the cross associated with theoperated horizontal bar. By bar conversational switches selectively to be enerthis arrangement, and whenthe contacts assogized in accordance with theline selecting digits ciated with the 'barsof one group are suitably of a directory number dialed at a calling sub In any form of automatic telephone system, in an automatic telephone system of the general there must necessarily be provided, in addition character briefly described above, wherein sepato the actual switching mechanisms per se, conrate communication and setting switches are uti- 50 trol apparatus for causing the switching mechlized in setting up desired connections between anisms'to be operated in the correct manner to the lines of the system and the communication carry out the desired, line selecting and line switches are of the exact form disclosed in appliswitching functions. This. control apparatus, in cants above-noted co-pending application. In

itsassociation with. the. respective numerical their broadest aspects, the objects as set forth 55.

above are realized in accordance with the present invention by providing a system comprising a plurality of main or conversational switches of the cross bar type, a plurality of automatic bypass or setting switches, and apparatus comprising the by-pass switches for causing the main switches successively to operate to set up a connection therethrough. Each of the main switches includes two sets of operating magnets, the magnets of one set in each switch individually corresponding to groups of lines and the magnets of the other set individually corresponding to the lines of the groups. At each switching stage a selected magnet in each of the two sets of magnets embodied in the selected crossbar switch is caused to operate under the control of an operatively associated by-pass switch so that the desired connection is successively advanced to the desired line.

More particularly the system comprises, at each intermediate switching stage, a plurality of main switches of the cross bar type, each hav-[ ing access to a plurality of lines divided into groups and each including two sets of operating magnets. Associated with the main switches is a by-pass selector switch corresponding to each! group of lines, one of which is arranged to be seized from the preceding switching stage. Each of the by-pass selector switches has associated therewith a tie switch having operating positions individually corresponding to the main switches and to the lines forming the group corresponding to the associated by-pass switch. Each of the by-pass selector switches includes control means operative in response to the seizure of the switch for causing the associated tie switch to operate to the position corresponding to a previously seized one of the -main switches and, also, apparatus operative in response to the operation of the tie switch to the position just mentioned for causing the operation of the magnet in the seized main switch which corresponds to the group of lines corresponding to the seized by-pass switch; A line testing or line selecting switch is provided ineach main switch which is arranged to respond to the operation of one of the group magnets in the same switch to select one of the lines of the group corresponding to the operated group magnet. Following the operation of the line testing switch of the seized main switch, means operative in response to the selection of one of the selected group of lines are caused to operate to energize the operating magnet of the seized main switch which corresponds to the selected line and to cause the operation of the tie switch, associated with the seized by-pass switch, to' a position corresponding to the selected line.

Further features of the invention pertain to the particular arrangement of the apparatus whereby the above and additional improved operating features are attained.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention,

both as to its organization and methodof op-/ ures 1 to 6, inclusive, illustrate the details of an automatic switch as disclosed in the above-mentioned co-pending application; Figs. '7 to 15, inclusive, illustrate an automatic telephone system having incorporated therein the features of the present invention; and Fig. 16 illustrates the manner of assembling Figs. 7 to 15, inclusive, to form a unified system.

Referring now more particularly to Fig. 1 of the drawings, there is illustrated the mechanical arrangement of an automatic switch constructed and arranged in accordance with the invention disclosed and claimed in the above referred to co-pending application. As shown, the switch mechanism is adapted to be supported on shelves such, forexample, as the shelves I3, l4, l5, l6

and I! extending between and secured to'side members H of a frame l2. Mounted on each of the enumerated shelves is a horizontal magnet, those for the shelves mentioned being indicated at l8, I9, 20, 2| and 22, respectively. Each of the horizontal magnets includes an armature upon which is mounted the horizontal member or bar for actuating the group of contact sets corresponding to the particular magnet. Thus, the magnets l8 to 22,inclusive, include, respectively, armatures 23, 24, 25, 26 and 21, upon which are mounted, respectively, the actuating bars 28, 29,30, 3! and 32. Each of these bars is guided in its movement, in the vertical plane by guide lugs mounted on the shelf carrying the associated magnet in a manner identical with that shown for the bar 28. The ends of the last-mentioned bar ride in slots provided in the upright portions of lugs 33 and 34, mounted at either end of the shelf l3. Each of the horizontal bars 28 to 32, inclusive, in cooperation with its respective associated horizontal magnet comprises means for actuating the contacts of an associated group of contact sets fromtheir respective normal positions to their respective elf-normal positions. Thus, the bar 28 has aSSOCi-r ated therewith a groupl of contact sets, four oil which are indicated at 35, 36, 31 and 38. If the sizel of the switch requires that a large number of 0011-! tact sets be placed under the control of each horizontal bar, two horizontal magnets may be required for actuating each bar, one magnet being positioned at each end of each bar. It will readily be apparent-how such additional magnets could be embodied in the structure shown. It

will further be apparent that the two magnets associated'with each bar could have their respective windings connected in multiple for simultaneous energization.

Mounted on the lower shelf I! is a group of vertical magnets, five of which are indicated at 39, 40-, 4!, 42 and 43, which individually include, in'the order named, armatures 44, 45, 46, 4'! and 48. to actuate a vertical bar associated with one group of vertical contact sets and guided in its movement by a pair of brackets projecting outwardly from the shelves l3 and I6. These brackets, comprising the pair for guiding the bar associated with the magnet 39 in its movement, are shown at M and 65. The bars associated with i the armature-s noted are indicated at 49, 5|], 5|,

52 and 53, respectively, and each thereof carries a plurality of projecting arms individually associated with one of the contact sets of the corresponding group of vertical contact sets.

Thus, the bar 49, which is under the control of the magnet 39, carries five arms, 54, 55, 56, 51 and 58, each of which projects inwardly and sidewise from the bar 49 so that, when the bar is pulled downwardly by the armature 44, all of the contacts included in the associated group of contact sets are moved from their respective normal positions to their respective off-normal Each of these armatures is arranged positions. The contact sets shown in association with the arms 54 to 58, inclusive, of the vertical bar 49 areindicated at 59, 60, BI, 62 and 63, respectively. i i i It will be understood that, when a switch of the above-described form, designed to have access to one hundred outgoing lines, is selectively operated to connect a line incoming thereto to any one of the outgoing lines, one hundred contact sets will be required in each of the plurality of groups, of contact sets associated respectively with the horizontal and the vertical bars. This necessarily means that tenvertical bars will be required, each arranged to actuate a group of ten contact sets, and ten horizontal bars will be reincluding the trunk and test conductors thereof must necessarily be multiplied between the corresponding contacts of all of the contact sets of one plurality of groups of contact sets. Thus,

if theincoming line is connected to the plurality of groups of contact sets associated with the horizontal bars, the conductors thereof must necessarily terminate at each of the contact sets actuated by each of these bars. The outgoing lines, on the other hand, terminate only at individual ones of the opposite contact sets or, in the assumed case; one line will terminate at each of the contact sets 59, 6H, 6!, 62, 63, etc., associated with the vertical bars.

Referring now more particularly to Fig. 2 of the drawings, there are illustrated the details of one of the horizontal magnets, such, for example, as the magnet 22 shown in Fig. 1,,and of one of the, spring assemblies associated with the corresponding horizontal bar 32 and the vertical bar 49. This magnet includes a magnetic core comprising a central leg 69 having a winding 10 wound thereon, a heel piece H, a side piece 12, and an armature 21. The Various contact spring assemblies, which individually comprise one contact of each of a pair of associated horizontal and vertical contact sets, are carried by an extension from the side piece: '12. Thus, one of the assemblies in the pair of sets associated with the horizontal bar 32 and the vertical bar 49 comprises springs M and 75 mounted on the side piece 12 and insulated from each other by an insulating member 18, and from the assembly screws by additional insulating members H as shown. The armature 21 ispivoted at 19 and carries the cross bar 32 for actuatingthe group of springs including the contact set of which the spring 15 is a part. This cross bar is insulated from the spring '15 by an insulating member 80. Each of the other contact springs associated with each of the horizontal bars is similarly insulated from its associated bar. The upper associated set of contact springs comprising the set, of which the spring '14 forms a part, is under the control oi. the arm 58 projecting from the vertical bar 49. This arm is insulated from the associated set of contact springs by an insulating member t i. Each of the other contact springs associated with each vertical bar is similarly insulated from the corresponding bar.

Referring now to Fig. 3, there is illustrated the vertical magnet 39 for actuating the vertical bar d9 associated with the contact set including the contact spring M. This magnet comprises a magnetic core having a central leg 84 having a winding 85 thereon, a heel piece 82, a side piece 83, and an armature 44. This armature is pivselection will be achieved. r

oted at 81 and is arranged to actuate its associated vertical bar 49] through a toggle member 88. As will be pointed out in greater detail here inafter, the number of springs included in the assembly of each of the contact sets associated withthe horizontal and vertical bars is dependent upon the particular arrangement of the control circuits for the. switch. Obviously, the spring,

assemblies may be designed to suit the requirements of any particular application. If additional contact springs are required for the pur-; pose of controlling the operation of the switch, such additional springs may be embodied in the magnet structure in any conventional manner.

In the operation of the switch as thus far described, and when one of the vertical magnets,

selected group. Following the energization of the switch. Thus, with the vertical magnet 39' and the horizontal'magnet 22 energized, only the contacts included inthe set of vertical contacts 63 will be in engagement with the contacts of the associated horizontal contact set, the remaining contact sets being disengaged due to the deenere gized condition of the other vertical andhorizontal magnets.

vertical magnet is energized and the same line An understanding of the mode of operation of the switch is facilitated by considering the sequence of contact spring movements illustrated in Figs. 4, 5 and 6, for the spring assembly M, l5, 15 associated with the horizontal magnet 22 and the vertical magnet 33. Normally, and with the two magnets noted deenergized, the springs 14,

15 and 'Hiare in theopen contact positions illustrated in Fig. 4. With only the vertical magnet 39 energized, the spring 14 is pushed down so that the contacts on the springs It and It make contact, but the contactsbetween the springs 14 and 15 remain disengaged, as shown in Fig. 5. This permits the line testing operations to be performed. Thereafter, and when the horizontal magnet 22 is energized, the lower spring 15 is pushed up so that the contact of this spring engages the contact of the spring it, as shown in Fig. 6. Thus, it will be seen that, with one hori zontal magnet energizedand one vertical magnet energized, the incoming .line to the switch is connected only to a particular one of the outgoing lines. Obviously, the various line testing contacts may, if desired, be arranged to close in response to operation of the associated horizontal magnets providing the horizontal magnets are used as the Obviously, the selected horizon tal magnet may be energized before the selected group select magnets and the vertical magnets are used to perform the line unit selecting operations.

Referring now more particularly to Figs. 7 to 15, inclusive, there isv illustrated an improved automatic telephone system of the by-pass type wherein automatic switches of the form just described are used as conversational switches to set up the desired connections between the various lines. In general, the system comprises a plurality of telephones or subscribers sub-stations, two of which are shown at A and B, adapted to be interconnected by means including their respective. subscribers lines 112 and i599, and switch trains including four of the aforementioned switches connected in tandem. These switch trains, when established, individually comprise a finder switch having access to the particular group of lines includin the calling line, a first selector switch individually associated with the finder switch delegated to the use of the calling line, a second selector switch accessible to the first selector switch and having access to a group of connector switches to which the called line extends, and finally a connector switch at which the called line terminates. Thus, in the system shown, the line H2 terminates in the contact springs of a finder switch Hill, which finder switch has individually associated therewith a first selector switch 200. The last-mentioned switch, in turn, has access to groups of second selector switches, each second selector switch of each group having the contact springs associated with its vertical magnets multipled to the corresponding contact springs of the other switches in the same group and connected to trunk lines outgoing to the connector switches, one of which is indicated at 400. These connector switches are also arranged in groups, the corresponding contact springs associated with the vertical magnets of the switches of each group being multipled together and connected to the same subscribers lines. Each of the lines extending to the various finder and connector switches of the system has associated therewith a line circuit which conventionally comprises line and cut-off relays. Thus, the line 112 has associated therewith a line circuit which includes a line relay R100 and a cut-ofi relay R'Hll.

As pointed out above, the various switches, corresponding to and including the finder switch I00, the first selector switch 200, the second selector switch 300, and the connector switch 400, are not directly operable to perform the line selectingfunctions, these switches being used' only to set up the conversational circuits involved in the various connections. For the purpose of controlling the conversational switches, there are provided by-pass switches arranged in groups, which bypass switches function to cause the conversa tional switches to route a'connection directly to a desired called line. Thus, there is associated with each group of one hundred lines a distributor switch, one of which is indicated at I50 associated with the group of lines terminating at the l finder switch I50. This distributor switch, which is of the rotary type, has access to a plurality of by-pass first selector switches, one of which is indicated at 250, which are arranged in groups and which, in combination with tie switches, one of which is indicated at I25, cause the conversational finder switches, such as the switch mil, to operate to select a calling line and to extend this line to the trunk leading to the associated conversational first selector switch. Each of the by-pass first selector switches further. has access to a number ofgroups of by-pass. second selector switches, one of which is indicated generally at 35%, which latter switchesfunction, in combination with tie switches, one of which is indicated at 225, to cause a selected conversational first selector switch to select an idle conversational second selector? switch. Each of the by-pass second selector switches also has access to certain groups of by-pass connector switches, one of which is indicated at 455, which latter switches, in'combination with tie switches, one or which is indicated at 325, function to control the operation of the conversational second selector and connector switches. The by-pass selector and connector switches embodied in the system are preferably of the well-known two-motion Strowger type.

Each of the conversational finder switches has associated therewith a small test switch which is operative to select a calling line from a subgroup of ten lines previously selected through the operation of one of the horizontal magnets of the associated finder switch. Thus, there is provided in association with the finder switch I a test switch H which is operative to test the lines of any selected subgroup of ten lines terminating at the switch Hill. In a similar manner, each of the intermediate selectorswitches has associated therewith a small test switch for selecting an idle trunk from a group of trunks selected through operation of one of the horizontal magnets embodied in the associated selector switch, the test switches associated with the intermediate conversational selector switches 205 and 300 being indicated at 215 and 315, respectively. Each of the conversational connector switches has embodied therein a directing switch which is utilized in energizing selected ones of the horizontal and vertical magnets embodied in the associated crossbar switch, the directing switch for the connector switch shown in Fig. being indicated at 415. switch further comprises switching and release relays, those embodied in the connector switch 450 being indicated at RI 530 and R1525, respectively.

As pointed out above each of the conversational switches I00, 200, 300 and 400 and the other switches corresponding thereto is of the form illustrated in Fig. l. The contact spring assemblies of the enumerated switches are slightly difierent from the spring arrangement shown in Figs. 1 to 6, inclusive, but the manner of altering the Fig. 1 arrangement to meet the operating requirements of the enumerated switches will be obvious to those skilled in the art. The line testing switches H5, 215 and 315, the directing switch 415, and the tie switches I25, 225 and 325 are all of the well known rotary type of which there are several well known commercial embodiments. Each of the rotary switches comprises a plurality of sets of bank contacts, wipers respectively associated with the contact sets and means comprising an operating magnet and a ratchet and pawl mechanism for driving the wiper over the contacts of their respective associated contact sets. The number of contact sets, the number of wipers and the number of contact points in the various switches varies in accordance with the requirements of each application. For example, the test switches I15, 2'15 and 315 each comprise two sets of contacts of ten points each, and two wipers. On the other hand the tie switch 225 comprises twelve sets of contact sets numbered froml28'i to I298 of one hundred points each, twelve wipers respectively associated with the contact Each conversational connector' sets and numbered from I228. to,l239, inclusive;

and an operating magnet MI'ZBB. The tie switch I25 comprises eighteen contact sets of one hundred points each, eighteen wipers and an operat ing magnet M750. Finally, the tie switch 325 comprises fourteen contact sets offone hundred points each, fourteen wipers, and an operating magnet Mlfififl.

As indicated above the conversational switches require a minimum of control relay equipment, only the second selector switches having included therein any substantial amount of control apparatus. More particularly, the second selector switch 30!] andthe other similar switches have individually associated therewith groups of relays arranged to perform the usual auxiliary functions incident to the completion of desired connections. The control equipment associated with the second selector switch 300 is illustrated in Fig. 12 and comprises a line relay Rl2l5 which is arranged to be held operated over a calling loop circuit extending to the switch 300, a battery reversing relay R1220 which is arranged to be held operated over the loop circuit extending to a called sub- .Operation of the system generally Neglecting for the present the details of the various control circuits and considering generally the operation of the system to set up a connection between two subscribers lines, such, for example,

as the lines llzand I599, on a call from the substation A to the substation B, the operation of the apparatus is initiated when the subscriber at the telephone A lifts his receiver from itshook; Assuming that the various switches illustrated are the switches utilized in setting up the connection, this operation on the part of the subscriber at the telephone A initiates the operation of the distributor switch I50 to select an idle by-pass first selector switch. Since it is assumedthat the switch 250 is the first switch available,

this switch is seized, and certain relays included therein are caused to operate to initiate the operation of the tie switch I25. This tie switch now functions to select the particular group of one hundred lines including the calling line by picking out the particular group of conversational finder switches, including the switch Hi0, at which the linesof this group of one hundred lines terminate. Following the selection of this group of conversational finder switches, the tie switch l 25 continues to operate to select an idle conversa tional finder-selector link; as, for example, the link comprising the finder switch Hill and the selector switch 200, in the selected group. After this idle link is selected, the horizontal magnet I-IM'IZU associated with the group of tenlines including the calling line, is caused to operate operatively to associate the ten lines with the test switch H5. The small rotary test switch H is next started into operation to select the particular calling line and to. cause the operation ofthe vertical magnet VM130, which corresponds to the calling line, thereby to connect this line through to the trunk line, extending to the con-' versatio-nal selector switch 200. The calling line is also extended by way of certain of the contacts of the two contact sets 158 and 159, embodied in the tie switch I 25, to the line conductors of the first by-path selector switch 259, whereby this selector switch is conditioned to respond to the impulses making up the first or thousands digit dialed at the calling, subscribers substation. When the calling line is switched through to the selector switch 255 the distributor H50 is released for further use. In response to the impulses of the first digit, the wipers of the selector switch 250 are stepped to a position corresponding to the desired group of by-pass second selector switches, which second selector switches correspond to a particular group of one thousand lines and to a particularone oi the horizontal magnets embodied in each of the conversational first selector switches. Following the selection of this group of Icy-pass switches, the by-pass first selector switch 250 automatically operates to select an idle one of the by-pass second selector switches in the selected group and to seize the selected switch. Followingthis operation, the tie switch 225 is caused to operate :to the position corre sponding to the seized conversational selector switch 260. When thetie switch 225 is operated to this position the operation thereof is arrested and the horizontal magnet I-IMlZliLcorrespondingto the by-pass selector switch 350 and to the group of one thousand lines including the called line, is caused to operate, thereby operatively to associate the trunks of the group of ten conversational second selector switches corresponding to HMIE'H) with the test switch 215. The test switch 215 is now started into operation to select 2 an idle switch as, for example, the switch ace, in the selectedgroup of conversational second selector switches. When the test switch 275 is operated to the position corresponding to the idle switch 300, the operation of the test switch is arrested and the vertical magnet VMI 28%, corresponding to the switchflllll, is caused to operate.-

350. is now held operated over the calling loop circuit and is conditioned to respond to the impulses making up the second line selecting digit dialed at the calling substation. Also, the p'reced-.

ing by-pass first selector switch 250 is released for further use. The by-pass second selector switch 35llrespondsto the impulses of the second digit dialed at the calling substation by selecting the particular group of by-pass connector switches having access to the called line and by also selecting the particular group of conversational connector switches having access to thecall ed line. At the end of thesecond digit the switch 350 automatically operates to select an idle one of the by-pass connector switches, as, for example,

the switch 450, of the selected group. i i

After the last-mentioned operation is completed, the rotary tie switch 325, individually associated with the selected by-pass connector switch 450, is started intooperation. When the wipers of the tie switch. 325fare driven ztothe position corresponding to the conversationahsecond selector switch 300, the operation ofthe tie switch is arrested and the horizontal magnet I started into operation. When the wipers of this test switch are driven to a position corresponding to an idle switch of the selected group, such, for example, as the switch 400, the operation of the test switch 015 isarreste'd and the vertical magnet VMI5I0, corresponding to the selected connector switch 400 is caused to operate. Following the operation of VMI 5"] the tie switch 315 is operated to the position corresponding to the selected conversational connector switch 400.

During the interval when the sequence of operations described i'n-the preceding paragraph are occurring the final two digits may be dialed at the calling substation. The by-pass connector switch 450 responds to the impulses of the I final two digits by operating its wipers to the particular position corresponding to the called line 5509, and in so doing rnarks for subsequent operation the particular horizontal and vertical magnets of the selected conversational connector switch which correspond to the called line. The arrangement is such that the called line is tested to determine its idle or busy condition only following the operation of the selected by-pass connector switch to the position corresponding to the called line and the operation of the tie switch 325 to the position corresponding to the selected connector switch 400. If the called line is found 'busy, the line relay RI2I5, the busy relay RI2I0 and the hold relay RI200 are caused to operate and busy tone current is transmitted back over the calling loop circuit to the receiver at the calling subscribers substation. d If, on. the other hand, the called line is idle, the line relay RI2I5,

the hold relay RI200, the booster battery control relay RI240, and the switching relays RI260 and RI530 are caused to operate. Substantially simultaneously with the called line testing operations the directing switch 415 is started into operation. As the wipers of this switch traverse the contacts of their respective associated sets, the respective horizontal and vertical magnets HMI 540 and VMI 550, corresponding'to the called line, are caused to operate. If the called line is idle, the two switching relays RI260 and R1530 are operated when the indicated horizontal and vertical magnets operate, so that ringing current is projected over the called line to energize the signal device provided at the called substation.

calling'party, although the battery reversing relay RI220, upon operating, functions to maintain the switches 300 and 400 operated independently of thehold relay RI 200 so that thesetwo switches are only-released-when the connection is cleared out at 'thecalled-substation.

The release of the conversational switch train is primarily underfthe control of the The control apparatus embodied in the lay-pass switches Referring now more particularly to the arrangement of the control relays and circuits included in the by-pass or control switches and considering first the link distributor switches, it is pointed out that the distributor switch I50, illustrated in Fig. 8 and common to one particular group of one hundred lines, comprises a ten-point rotary switch including thirteen sets of bank contacts, thirteen wipers and means comprising an operating magnet M830 and a ratchet and pawl mechanism, not shown, for driving the wipers over the contacts of their respective associated sets. The distributor switch further comprises a start relay R800 which is operative in response to the operation of anyv one of the line relays associated with the lines of the group corresponding to the switch I50, and a slow-to-release hold relay R820. Also embodied in the switch I50 is a fast-acting test relay R8I0 of the polarized type which is arranged to operate when the associated rotary switch is operated to a position corresponding to an idle by-pass first selector switch and functions to arrest the operation. of the rotary switch and to cause the operation of the line and hold relays embodied inthe selected by-pass switch.

The selectorswitch 250 comprises a Strowger switching mechanism including a vertical magnet MIMD, a rotary magnet MI042, a release magnet MIMI and off-normal springs I043, I044,

I045, I046, I048, I049 and I050, all arranged to operate in the manner set forth in detail hereinafter. The relay equipment embodied in this selector switch comprises a line relay RIillI), a slow-acting hold relay RI020, a slow-acting pulse circut control relay RI030, and a fast-acting test relay RI 0000f the polarized type which functions to terminate rotary movement of the Strowger switch wipers during a trunk hunting operation. The relay equipment further comprises the group of relays R900, ROI0, R920, R930, R940, R050 and R960, illustrated in Fig. 9, which are operative to control the operation of the tie switch I25 and of a selected conversational finder switch. More particularly, the shaded pole alternating current relay R050 is operative to condition the test relay R9l0 for operation when the tie switch I25 is operated to a position corresponding to a group of conversational finder switches having access to the calling line, the relay R0|0 is operative to arrest the operation of the tie switch I25 when it is operated to a position corresponding to an idle finder switch of the selected group, and the relay R900 is operative in response to operation of RSIO to complete the operating circuit for the particular horizontal magnet of the selected finder switch which corresponds to the subgroup of tenlines including the called line. The relay R020 is operative to condition the relay R940 for operation when the relay R900 operates, and also to initiate the operation of the test switch, such, for example, as the switch I15, embodied in a selected conversational finder switch. The test relay R940 serves to arrest the operation of the test switch embodied in a selected finder switch when the test switch is operated to a position corresponding to the calling line and also to cause the operation of the parcorresponds to the calling line. The relay R040 is also operative to cause'the operation of R900,

which latter relay functions as a test circuit transferrelay. The slow-to-release relay R930 is provided for the purpose of conditioning the test relay R940 for operation and for holding the preceding distributor switch operated. The relay R930 also functions to prepare the operating circuit for a selected one of the vertical'magnets of the selected finder switch. Alternating current energy for energizing the alternating current relay R960 is derived from an alternating current source, not shown, and is delivered to the relay R050 through a voltage stepdown transformer 910.

The by-pass second selector switch 350, illustrated in Figs. 11 and 13, is similar to the first selector switch 250 in that it includes a Strowger switching mechanism having embodied therein a vertical magnetll/II3I0, a rotary magnet MISII, a release magnet MI3I3 and off-normal springs I314, I315, I315, I3'I'I,IIl6,II'II and H18. The control equipment of this selector switch includes a line relay RIIBO, a slow-acting hold or release relay RI3I0, a pulse circuit control relay RI330 and a fast-acting trunk test relay Rl300. of the polarizedtype. There is also provided a group of relays RII00, RIIIO, Ril20,

RII30, RII40, RII50 and RII60 which are arranged to control the operation of the tie switch 225 and the operation of a conversational first selector switch selected through operation of the tie switch 225. Of the enumerated relays, the shaded pole alternating current relay RI I40 is arranged to operate to arrest the operation of the tie switch 225 whenthe tie switch is operated to a position corresponding to a selected one of the conversational first selector switches,

and also to cause the operation of the horizontal magnet in the selected conversational selector switch which corresponds to the by-pass switch 350. The relay RI I40 is arranged to restore following, the operation of the selected horizontal magnet in the selected conversational first se lector switch. The two-step relay RI I30 is arranged to respond to the operationand restoration of the relay RI I40 by initiating the operationof the test switch associated with the selected conversational first selector switch. The two-step relay RH 30, in cooperation with the slow-to-release relay RIIII), also functions to condition the fast-acting polarized relay RI I20 to operate when the test switch embodied in the selected conversational first selector switch is op erated to a position corresponding to an idle conversational second selector switch in the selected group. As indicated, the relay RIIZIl is provided for the purpose of arresting theoperation of the test switch in a selected conversational first selector switch and for the further purpose of energizing the particular vertical magnet of the selected conversational first selector switch which corresponds to the conversational second selector switch selected by the test switch embodied therein. In addition, the relay RII20 functions to initiate a second operation of the tie switch 225, the second operation being arrested when the tie switch is operated to the position corresponding to the selected conversational second selector switch through a second operation of the alternatingcurrent test relay RI I 40, Finally, the relay RI I is arranged to respond to the second operation of RII40 and functions to maintain the partially completed conversational switch train operated and also to switch the calling loop circuit through to the control apparatus of the selector switch 350. 'lh.ee s ls li .ent 0. .thecontrol .anparatuslpro- I nector switches.

vided in each of the by-pass second selector switches is such that all line testing and certain ofthe other auxiliary control functions are performed therein rather than in the by-pass con- More particularly, the control of the tie switch associated with any selected bypass connector-switch is primarily achieved by operation of certain of the relays embodied in the by-pass second selector switches. Thus, there are provided in the by-pass selector switch 350 four relays RI I50, RI340, RI350 and RI366 which are arranged to cooperate with certain of the relays provided in a by-pass connector switch selected by the switch 350, to control the operation of the tie switch associated with the selected by-pass connector switch and also the operation of the conversational second selector switch which is ultimately selected through the operation of the tie switch. The relay RI I60, in cooperation with 0 two additional relays RI I I0 and RI 320, also functions to test the, condition of a selected called line. More particularly, the relay RI I20 is a busy relay and is arranged to operate only when a selected called line is found to be in a busy condition. The relay RI I70, on the other hand, is an idle testrelay and is arranged to operate only when a selected called line is found idle.

The by-pass connector switches, like the bypass selector switches, are of the well-known Strowger type, that indicated at 450 being illustrated as comprising a Strowger switching mechanism, a vertical magnet MI424, a rotary magnet MI426, a release magnet MI409, and. 01T- normal springs I408 and I4I5. The relay equip ment in this connector switch comprises a start relay RI400 which is operative in response to the seizure of the switch by a preceding by-pass selector switch. This equipment further comprises a slow-to-release series relay RI420 arranged in a conventional manner and three relays RI4I0, RI430 and RI440 which are arranged to cooperate with the previously mentioned group of relays embodied in the preceding second selector switch 350, or a sirnilarswitch, to control the operation of the tie switch 325 and the operation of a conversational second selector switch selected through operation of this tie switch.

The Strowger switching mechanism embodied in the connector switch 450 is of somewhat special construction in that it is provided with a set of contacts I465which are arranged successively to be engaged by a wiper I 4' during the vertical movement of the wiper carriage structure embodied in the Strowger switching mechanism, and a second set of contacts I466 whichare arranged successively to be engaged by a second wiper i412 during rotary movement of the wiper carriage structure. The arrangement of the con tacts in the two sets is such that the wiper I4II engages a selected one of the contacts in the set I455 irrespectiveof the rotary position ultimately attained by the wiper carriage structure during a line selecting operation. Only one of the contacts in the set I466, on the other hand, is engaged by the Wiper I412 with this wiper standing in any particularly rotary position, although the arrangement i'ssuch that the contacts of the set I466 may be engaged by the wiper I 412 with the wiper carriage structure occupyingany one of its vertical operating positions. The added contact sets, together with the wipers respectively associated therewith, are arranged to cooperate with a small rotary marking switch I460 for the purpose of selecting for energization the particular horizontal and vertical magnets of a selected thereto.

1 circuit for energizing the line relay R100. circuit extends from ground at RA'I I2, by way of conversational connector switch which correspond to a selected called line. This rotary marking switch comprises two sets of contacts l iiii and M02, wipers I003 and i064 respectively associated with the two contact sets, and an operating magnet .MMM for driving the wipers over the contacts of their respective associated sets. The switch 050 further comprises two sets of off-normal springs M and M56, the springs of each set being arranged to be moved into engagement when the. wipers of theswitch M00 are moved to their respective first off-normalpositions. Energizationof the magnet MMBA is under the control of a sloW-towperate relay BMW, which relay, in turn, is arranged to be energized only in response to the operation of a two-step relay EH50. The relay Rltiil' also functions to complete the testing circuits by Way of which a selected called line is tested to determine the idle or busy condition thereof. This relay is arranged to be energized in response to the operation and restoration of a slow-acting relay Bidet which is energized in parallel with the rotary magnet Ml l'iii during the final series of impulses transmitted to the connector switch 450. For the purpose of maintaining the by-pass switch trainoperated during the interval when the marking switch M00 is operating to cause the operation of the selected horizontal and vertical magnets of a selected conversational connector switch, the relay EH50 is arranged to maintain ground on the control conductor of the trunk incoming to the connector switch 150 for the duration of the indicated period. This ground is removed to'permit the release of the bypass switch train through the provision of a second two-step relay Ri ltli whichis arranged partially to operate when the wipers of the switch I000 are stepped one step oii' normal and completes its operation when the wipers of the switch M00 havetraversed all of their contacts and are returned to thehome position thereof.

Operation of the apparatus to find a calling Zine Referring now in greater detail to the operation of the system to establish a connection between the substations A and B, and assuming that the apparatus illustrated is used in setting up the connection, operation of this apparatus is initiated when the calling party at the substation A removes his receiver from its book to complete a This CI'l l', the bridge placed across the line conductors CW3 andC'l'l i at the substation A, CI'I3, RAIII and the winding of R100 to battery. The relay R100, upon operating, completes, at RA'IOI, a path extending through the Winding of the cutoff relay Rllt for applying battery potential tov the contact 323 of the switch I00, thereby tomark the calling line H2 in the subgroup of ten lines extending to the contact springs controlled by the horizontal magnet HlVi'lZll. At RA'I02, the relay R100 applies ground to the marking conductor 0773!, this conductor being common to the particular subgroup of ten lines which includes the calling line H2, and serving to identify the ten line subgroup which includes the calling line in the group of one hundred lines extending to the finder switch 500 when groundis applied From the above description, it will be apparent that the conductor C'I8I is multipled to, the contact springs corresponding to the springs associated with RATI02 and respectively embodied in the ten, line. relaysrespectively associated with 'line E72.

the lines of one ten line subgroup. The remaining nine subgroups in a group of one hundred lines each has associated therewith an independent marking conductor, corresponding to 078i, and terminating at the contact springs of the various line relays individually associated with the lines of the other subgroups. All of the marking conductors extend to contact springs of the start relay R800 embodied in the by-path distributor switch 550.

At RA'J03, the line relay R700 completes a circuit for energizing the start relay R800 in the by-path. finder distributor switch I00, which switch, as was noted above, is common to the one hundred lines terminating at the finder switch 800. The circuit noted extends from ground at RAYM, by way of RA'I03, the start conductor C'i82 and the winding of R000 to battery.

The relay R000, upon operating, completes, at RA00l, an obvious circuit for energizing the slowi to-release relay R820; prepares, at RA802, a circuit for energizing R050; and prepares, at RAi 03, a circuit, traced hereinafter, for energizing the horizontal magnet HM'I20, associated with the subgroup of ten lines which includes the calling At RA000 and the remaining eight armatures of R800 circuits are prepared for energizing the other nine horizontal magnets of the switch 500. The relay R020, upon operating, completes, at RA02I, an obvious circuit for enerl gizing the lower winding of the polarized relay Rail This last-mentioned relay-does not operate in response to the energization of its lower winding, it being so constructed that it operates, only when both of its windings are energized, l but, following its operation, is held operated so long as its lower winding is maintained energizeda The energization of the lower windingof Rill0 does, however, render this relay'exceedingly fast to operate when its upper winding is subsequently energized. At RABZI, the relay R820 also completes a circuit for energizing M830, this circuit extending from ground at RA82I byway of RAOI3, the circuit-interrupting contacts associated with MA03I, and the winding of M830 to battery. The magnet M830 now starts to operate, buzzer fashion, to drive the associated wipers 831 to I, inclusive, to a position corresponding to an idle by-path first selector switch. The idle or busy condition of any by-path first selec- 5 tor switch, such, for example, as the by path first selector switch 250, is determined by the nature of the potential existing on the control conductor embodied in the trunk line extending thereto. Thus, when the switch. 250 is idle, battery is 4 extended through the upper winding of R I 030, the oil-normal springs I046, C015 and RA922 to the control conductor C800 extending to the switch 250 and terminated at a contact in the control contact set 833 of the finder switch I50. When, on the other hand, this by-path switch is busy, the control conductorC848 is either grounded or open-circuited. Assuming that the switch 250 is the first idle by-path first selector switch available, when the wipers'831 to GM, inclusive, are driven to the position corresponding to this switch, a circuit is completed for energizing the upper winding of R8) and the upper winding of the slow-to-release relay RI030 in series. This circuit extends from ground at RA82I by way of i the upper winding of R8l0, C842, the wiper 838,

C848, RA022, C915, the off-normal springs I046, and the upper winding of RI030 to battery. The relay RBIil operates immediately its upper winding isenergized. over theabove-traced circuit and 7 interrupts, at RABI 3 and RC8 Ii5,,the above-traced operating circuit forM830, thereby toarrest the stepping operation of thewipers 831 to, 841, inclusive. At RA8I3 and R0814, the relay. R810 short-circuits its upper winding, thereby to render itself slightly slow, to release. As indicated above, R8I0 is held operatedso long as its lower winding is maintained energized. At RA8I2, the relay RBIO opens a point in the operating circuit for Rtiifland, at RA8I I, it completes a circuit for energizing the line relay RIOI0, this latter circuit extending fromground by way of RABI I, the switch wiper 831, C841, C913, RAIOI, RCI002, and the upper winding of RIOI0 to battery. When the line relay RI 0 I 0 operates, it completes, at RAID and RC'IDIZ, a circuit extending from ground at RAI009" for energizing the slow-to-release hold relay RI020, which latter relay operates to apply, at RAI02I, ground polayR820 restores to open, at RA82I, a point in the above-ltraced operating circuit for R8I0 and RI030. At RAI022, the relay R|020 completes an, obvious circuit for energizing the lower winding of RI000. The last-named relay does not operate at this time, however, it being so constructed and arranged that it operates only when both of its windings are energized. The relay RI000 is rendered exceedingly fast to operate when its lower winding is energized, and, is held operated, following its operation, so long as its lower winding is held energized. At RAI023, the hold relay prepares a test circuit, traced hereinafter, for energizing the upper winding of RI000. At RAI025, the hold relay prepares a circuit, traced hereinafter, for energizing the vertical magnet M1040 of the by-path selector switch 250 in accordance with the impulsesmaking upthe first i the upper winding of the polarized relay R930,

- this circuit extending from ground through RAIHM to C911 where it divides, one branch extending by way of RA941 and the winding of I R930 to battery, and the other branch extending by way of the upper winding of R940 to battery. The relay R930 now operates to prepare, at RA933, a test circuit traced hereinafter for energizing the lower winding of R940. The relay R940 does not operate when its upper winding is energized over the circuit just traced, it being in this respect similar to each of the two relays R8 i 0 and RI 000 in i that it is rendered exceedingly fast to operate when its upper winding is energizedbut only operates when both of its windings are energized.

I The hold relay RI020, upon operating, also completes a circuit for energizing the operating magnet M150 of the tie switch I25, this circuit extending from ground at RAIIIZ I by way of 0911, RAMS, RAM-I, RA9II, C183, the circuit- 1 interrupting contacts associated with MA15I and the winding ofM150 to battery. By virtue of the circuit-interrupting action of the contacts controlled by MA15I, the magnet M150 now operates, buzzer fashion, to drive the wipers of the switch I25 over the contacts of their respective associated sets. The function of the switch I25 is two-fold. The first step in its operation is that of selecting the particular group of conversational finder switches at which are terminated the lines of the nonemundred line group including the calling line 112. Following this op: eration, the switch I25 is, arranged to operate to select an idle one of the selected group of con-, versational finder switches.

Thus, when the wiper 162 is driven by the magnet I M150 to a point where it engages the first ofthe contacts in the set 152 terminating the extra control conductor C185, a circuit is completed for Any desired num-f ber of conversational finder switches may be inenergizing R950. I This circuit extends from one terminal of the transformer secondary winding MI by way of RASBI, R0952, the winding of R950, RA923, C185, the wiper 102, C184, RA002',

the wiper 839, C044, RA9 I5, RC965 and RA964 to the opposite terminal of the transformer winding 911. When energized over the abovetraced circuit, R950 operates, to prepare, at RA95I, a circuit, traced hereinafter, for en-i ergizing the test relay R9I0. It will be observed that the nine contacts of the set 152 following that terminating C184 are multipled together and to the contact terminating C100. Thus R950 is held operated during the continued operation of M further to advance the wipers of the tie switch I25. During continued operation of the tie switch I25 from the position corresponding to the first contact terminating C184, the con versational finder switches of the selected group are tested to, determine the idle or busy condi; tion thereof. Thus, if the switch I 00 is idle, battery'is extended by way of the resistor 131, the

contact 136, and the armature 134 to the control conductor C119 of the link line 116. On the other hand, if this switch is busy, ground is preent on the control conductor C119. '80 long as ground is found by the wiper 110 during the continued operation of the tie switch I25, no circuit is completed for energizing the test relay R9I0.

When, however, an idle switch is found, .a circuit is completed for energizing the test relay RQII). Thus, assuming that the first switch I00 in the selected groupis idle, the circuit for en'- ergizing R9I0 is completed immediately the wiper 110, is operated into engagement with the contact of the set corresponding to this switch. This circuit extends from ground at RA95I by way of RA921, the lower winding of R9I0, C198, the wiper 110, C119, the armature 130, the'contact 136, and through the resistor 131 to battery. When energized over the above-traced circuit, R9I0 first operates to complete, at the X contacts associated with RA9I3, a holding circuit for itself, this circuit extending from ground at RAIIJZ I by way of C911, RA9I3 and the upper winding of R9I0 to battery. At RA9I I, the relay R010 interrupts the above-tracedoperating circuit for M150, thereby to arrest the operation of the tie switch I25 with the wipersthereof standing in the position corresponding to the selected the switch by another one of the by-path first selector switches. At RA9I2, the relay R910 completes a-circuit for energizing R900, thiscircuit extending from ground at RAI024 by way of C911, RA946, RA9I2 and the winding of R900 to battery.

The relay R900, upon operating, completes a circuit for energizing the particular horizontal magnet which is associated with the ten line subgroup including the ealling line 112. In this regard, it is emphasized that the line relay R100 associated with the calling line 112, upon operating, prepares a circuit for energizing only the horizontal magnet of a selected conversational finder switch which corresponds to the ten line subgroup including the calling line. In the particular case under consideration HM120 is energized, the circuit for this magnet extending from ground at RA1I4 by way of RA102,C-18I, RA803, the wiper 840, C845, RA90I, C188, the wiper 104, C189, and the winding of HM120 to battery. It will be apparent that, had the calling line been included in a diiferent ten line subgroup, the line relay associated with this line would, upon operating, have prepared a circuit for energizing a different one of the horizontal magnets. As indicated previously, since each group of one hundred lines is divided into ten smaller sub-groups of ten lines each, ten hor izontal magnets'are provided in each conver I embodied in the by-p'ath finder distributor switch I50 includes a set of contacts for each of the subgroups, as does also the rotary switch embodied in the tie switch I25. In order to simplify the drawings, however, only two of the contact sets provided for this purpose are illustrated, namely, those for the. horizontal magnets cor responding to and including I-IM120, and those for "the horizontal magnets of the group associated with the last ten line subgroup of the one'hundred-line group.

When the horizontal magnet I-IM120 operates, it completes, at its armature 125, a holding circuit for itself, this circuit extending from ground, asJextended over the above-traced path, to the I control conductor C119, by way of the armature 125 and the winding of HM120 to battery. The magnet HM120 also operates all of its associated line switching contact springs corresponding to and including the springs 12I, 122 and 123. Through this last-mentioned operation, the calling line is marked in the bank contacts of the rotary switch I15, the marking path extending from battery at the lower terminal of the cut-off relay R1I0 by way of the winding of R1 I 0, RAI, .the'contact springs 123 and 124 to the third contact of the set 143. Following the operation of I-IM120, the switch I is started in operation to search'for the particular calling line of the selected ten line subgroup. Thus, simultaneously with the completion of the operating circuit for HM120 as traced above, there is completed'a circuit for energizing R920, this latter circuit extending from ground at RA904, by way of RA926, and the lower winding of R920 to battery. The relay R920, upon operating, first completes, at RA925, a holding circuit for itself, this circuit extending from ground at RAI024 by way of C911, RA925 and the upper winding of RA920 to battery. At RA923, the relay R920 opens a point in the above-traced circuit for energizing R950, causing the last-mentioned relay to restore to. open, at RA95I, a point inthe testcircuit for energizing R9I0. "At RA924, the hold relay R920 opens a further point in the operating circuit for M150. At RA926, the relay R920 opens a point in its operating circuit as traced above. At RA928, the relay R920 prepares a test circuit, traced hereinaftenfor energizing the lower winding of the test relay R940. At RA921, the relay R920 opens a further point in the test circuit, traced above and including the lower winding of R9I0. At RA92I, the relay R920 completes a circuit for energizing M140 embodied in the rotary switch I15, this circuit extending from ground by way of RA92I, RA94I, 0194, the wiper 101, G195, the circuit interrupting contacts associated with MA14I, and the Winding of M140 to battery. By virtue of the circuit interrupting action of the contacts associated with MA14 I, the magnet M140 operates buzzer fashion to drive the wipers 144 and 145 over the contacts of their respectiveassociated sets 142'and 14 3. When the wiper 145 is driven into engagement with the marked contactcorresponding to the calling line 112, the above-mentioned test circuit is completed for energizing the lower'winding of the test relay R940. In the present case, this circuit extends from ground at RA928 by way of the lower winding of R940, RA933, 0192, the switch wiper 10 6, C193, the wiper 145, the third contact of the set 143, the springs 124 and 123, RA10I, and through the winding of R1I0 to battery. When its lower winding is energized over the above-traced circuit, the relay R940 immediately operates to open, at RA94I, a point in the above traced operating circuit for M140, thereby to arrest the operation of the switch I15 with the wipers thereof standing in the position corresponding to the calling line 112. At RA943, the relayR940 completes a circuit for energizing the vertical magnetcorresponding to the calling line, namely, the magnet VM130, this circuit extending from ground at RA93I by way of RA943,' C190, the switch wiper 165, C19I, the switch wiper 144', the third contact of the set 142 and the winding of VM130 to battery. When energized over the above-traced circuit, VM130 attracts its operating armature to actuate all of the switchsprings associated'therewith and in so doing moves thesprings 13I, 132 and 133 into engagement with the springs 12I, 122 and 123,

vided in the by-path finder distributor switch I50.

When R940 operates, it functions to cause the release of the distributor switch I50. Thus, this relay, upon operating, opens, at RA946, a point in the operating circuit for R900 and, at RA941, a point in the operating circuit for R930. At RA945, the relay R940 opens a further point in the test circuit, described previously, by way of which R950 was energized. At RA948, the relay R940 completes an obvious circuit for energizing the lower winding of R960. Upon operating, the relay R960 completes a holding circuit for itself, this circuit extending from ground at RAI024 by way of C911, RA961 and the upper winding 

